1. Field of the Invention
The present invention relates generally to methods of controlling transmissions of vehicles and, more particularly, to a technique for preventing a shift shock from occurring when shifting gears in response to deceleration of a vehicle that is provided with an AMT (automated manual transmission) including a DCT (double clutch transmission), etc.
2. Description of Related Art
A process of downshifting when a vehicle provided with a conventional DCT decelerates will be described with reference to FIG. 1.
When the vehicle decelerates, a transmission controller drives an actuator so that a corresponding synchronizer is operated to achieve gear engagement of a target gear that is lower than the current gear.
Typically, the actuator linearly moves a sleeve of the synchronizer to conduct synchronization and engagement of a related shift gear.
As stated above, when the actuator begins to operate, the sleeve compresses a synchronizer ring towards a clutch gear of the target gear by means of a key so that the synchronization begins (at a point of time T1 of FIG. 1). A second input shaft which engages with the shift gear of the target gear is synchronized with an output shaft, and the rpm of the second input shaft thereby increases.
During this process, an increase of the rpm of the second input shaft results from transmission of rotating force of the output shaft to the second input shaft through the synchronizer. This means the torque of the output shaft is used to increase the speed of the inertial mass pertaining to the second input shaft. Due to the torque transmitted from the output shaft to the second input shaft, backlash of elements, such as gears, etc., from the second input shaft to the output shaft are arranged in one direction.
After, the synchronization has been conducted as described above, the rpm of the second input shaft becomes equal to that of the output shaft. Subsequently, when the sleeve is further moved by the operation of the actuator, the sleeve passes over the synchronizer ring and the key. Thereby, the compression force with which the sleeve pushed the synchronizer ring is removed, resulting in a momentary breakdown of the synchronization (from T2 to T3).
Thereafter, when the sleeve is further moved by the operation of the actuator, the sleeve engages with the clutch gear of the target gear, thus completing the gear shifting (after T3).
During breakdown of the synchronization, if there is no frictional resistance induced as a result of the rotation of the second input shaft, the second input shaft is in free rotation and thus maintains the rpm when in the synchronized state. The rpm of the output shaft reduces because the vehicle is decelerating. As a result, just before the sleeve engages with the clutch gear, the rpm of the second input shaft exceeds that of the output shaft.
In this state, if the sleeve just moves and engages with the clutch gear, torque is applied to the elements in the direction in which the second input shaft that is faster than the output shaft rotates the output shaft. Thus, the backlash of the elements that have been arranged in one direction when the synchronization begins are rearranged in the reverse direction. Because of such a phenomenon, the elements, such as gears, between the second input shaft and the output shaft that have been rotating at high speeds collide with each other, thus causing shock.
Such shift shock and noise which are induced during the above-mentioned process of the conventional technique deteriorate the quietness of the vehicle, causes a user discomfort, and reduce the marketability of the vehicle.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.